Dear @Kevin.Chang.
My understanding is that the SoilDyn module has a placeholder for a distributed springs (e.g., P-Y curve) model, but that this feature has not been fully developed. If you were to use a distributed springs model (not available without a source code change), you would have to model the pile in SubDyn as a structural member to its free end below the seabed. However, SoilDyn only currently supports a coupled springs representation of the foundation, including through simple linear matrices or through coupling to the REDWIN superelement. To model a coupled springs foundation, you should only model hte pile in SubDyn as a structural member down to the seabed.
Best regards,